Two experiments were carried out in healthy human volunteers in order to investigate the effect of novel experiences on retrieval, and the influence of naltrexone thereupon. Naltrexone (50 mg) and placebo (50 mg of starch) were given orally using a double blind design. In Experiment 1, the subjects were asked, on two consecutive days, to recall well-known facts or events, and to recall the year in which major events took place. On Day 2, some subjects were, and others were not, exposed to a nonsense text prior to testing, which was viewed as a novel experience by the subjects. Exposure to the text was followed by enhanced scores in both memory tests. The effect was blocked by naltrexone, but not by the placebo, given 1 hr prior to the novel experience; the treatments had no effect of their own in subjects unexposed to the nonsense text. In Experiment 2, the memory tests were the recognition of famous faces, and the dates test (see above); and the novel experience was being taken for 5 min to a room where they had never been before. Again, the novel experience was followed by increased scores in both memory tests in the untreated and placebo groups, but not in the naltrexone treated subjects. These results confirm previous findings on memory enhancement by pre-test exposure to novel experiences, and suggest that endogenous opioid, or at least naltrexone-sensitive, mechanisms are involved in the effect.